The desirability of drying air after compression is well known. A variety of systems have been developed over the years for this purpose, many of which employ two beds containing an adsorbent desiccant material such as activated alumina. In such systems the beds alternatively dry the process stream and then are regenerated either using an auxiliary source of heat in so-called heat-reactivated systems, or through conservation and use of the heat of adsorption for effecting regeneration, in so-called heaterless systems. In heat-reactivated systems, it has been common to use fixed cycles several hours in length, such that a bed is on adsorption for a period of time, perhaps eight hours, after which it is regenerated during the eight hour adsorption cycle for the other bed. In the heaterless systems relatively short, fixed cycle times are usually used, typically about four minutes for drying with four minutes allowed for regeneration while the other bed is drying the process stream. Mechanical cam timers operating solenoids which in turn operate the flow control valves have typically been employed for the control of such fixed cycle systems.
Typically both heat-reactivated and heaterless systems are sized for maximum contemplated loading conditions, a condition which may seldom or never be encountered during actual operation. As a result, at the end of a fixed drying cycle a bed may have adsorbed only a fraction of its moisture capacity. In the case of heat-reactivated dryers, such underutilization of the bed while on adsorption coupled with a full energy regeneration can seriously affect overall operating efficiency. Moreover, in the case of heat-reactivated dryers which employ resistance heaters within the beds, sometimes referred to as internal heat-reactivated dryers, regeneration with only a partial moisture load on the bed can result in excessive heating of the desiccant, which not only tends to degrade the desiccant but can also be dangerous.
Similarly, off-peak operation of heaterless dryers on fixed cycles can result in low operation efficiencies. By their nature heaterless dryers employ approximately 15% of the dry product air for regeneration purposes such that unnecessarily purging a partially loaded bed for the full regeneration period is highly inefficient.
Several systems have been suggested in recent years using "demand" rather than fixed cycling for both heat-reactivated and heaterless dryers. U.S. Pat. No. 3,448,561 discloses a system for sensing the actual moisture content of the on-stream bed and terminating the adsorption cycle when the bed is saturated. Alternatively, the patent suggests that the moisture content of the regenerating bed can be monitored, terminating the purge flow when the moisture content of the bed has dropped below a predetermined minimum. That patent discloses the use of a lithium chloride humidity sensor external to the desiccant bed with gas sampling conduits leading from the beds to the cell.
Another method which has been suggested for determining the moisture loading of a bed involves the placing of a capacitor directly into an adsorbent bed with adsorbent between the plates. As the moisture loading varies, the dielectric constant of the adsorbent changes, such that the capacitance provides an electrical indication of the moisture loading. Such a capacitance sensing arrangement is disclosed in application Ser. No. 267,964 filed May 27, 1981.
In one system which has been developed for the use of a capacitance as disclosed in the co-pending application, a periodic electric potential is applied to the capacitance probe so that its response is a function of the capacitance. A frequency to voltage converter generates a moisture indicating voltage proportional to the capacitance of the capacitance probe. Typically a differential amplifier compares the moisture indicating voltage to a threshold voltage to generate a binary signal indicating that the desiccant is fully regenerated. The moisture indicating voltage is also compared to high and low threshold voltages defining a range of moisture content from 0 to 100% saturation of the desiccant. Any voltage out of this range is assumed to be due to sensor malfunction and hence the "window comparison" provides a failure signal.
The prior art moisture sensor comprising the capacitance probe and the electronics for performing a frequency to voltage conversion and threshold comparison function performs its intended function of generating a binary signal indicating the end of required regeneration, but it does so using rather sensitive and "touchy"analog circuits. The analog circuits require stable reference voltages and are sensitive to component aging, tolerances and drift. In order to eliminate undesired noise on the moisture sensing voltage the moisture sensing voltage analog signal is typically low-pass filtered at a time constant of approximately ten seconds. This time constant slows down the response of the sensor and the rather high resistance and capacitance component values for providing the ten second time constant are undesirable. The one binary bit of moisture indicating information provided to the air dryer control system by the moisture sensor is a rather small portion of the information monitored by the sensor, but it is just about all of the information that the prior art control system can use.
As suggested in the co-pending application referenced above a binary signal indicating that the desiccant is fully regenerated may be used in conjunction with the flow valve control system with either heat-regenerated or heaterless dryers to result in "demand" cycling.
In addition to the deficiencies noted with the previously suggested capacitance-monitoring system, known controls for valving to alternatively shift flow between desiccant beds have numerous disadvantages. As already noted mechanical cam timers have typically been employed to provide fixed length cycles. Such mechanical timer circuits have recently been adapted for use with capacitance sensing arrangements of the type discussed above to provide "demand" cycling in both heat-regenerated and heaterless dryers by inhibiting certain of the timing signals. Such mechanical timer based systems tend to be bulky, offer little or no "reprograming" flexibility and can be difficult to accurately adjust.